Nonaqueous electrolyte secondary batteries (mostly lithium-ion secondary batteries) that use a carbonaceous material as the anode active material and a layered oxide containing nickel, cobalt, manganese, or the like as the cathode active material have already been put into practical use as the power supplies for a wide variety of fields ranging from small-sized electronic devices to large-sized electric vehicles and the like. The user demands for even smaller size, lighter weight, and longer operating time and life are strong, and there is an increasing demand for higher battery densities and capacities and higher repetitive performance. However, a conventional carbonaceous material has a limit in increasing its charge/discharge capacity. Also, low-temperature baked carbon that is considered to have a high capacity is low in material density, and therefore, it is difficult to increase the charge/discharge capacity per unit volume. Therefore, to realize high-capacity batteries, development of a new anode material is necessary.
The use of a single metal such as aluminum (Al), silicon (Si), germanium (Ge), tin (Sn), or antimony (Sb) as the anode material for achieving a higher capacity than a carbonaceous material has been suggested. Particularly, where Si is used as the anode material, a high capacity of 4200 mAh per unit weight (1 g) is obtained. However, an anode made of such a single metal repeatedly absorbs and releases Li. Therefore, microscopic pulverization of elements occurs, and excellent charge-discharge cycling characteristics cannot be achieved.
So as to solve the above problems, a tin oxide or silicon oxide in an amorphous state may be used to achieve a high capacity and excellent cycling characteristics at the same time, and, in combination with a carbonaceous material, further improvement can be realized. However, even if an improved, high-capacity tin oxide or silicon oxide is used, the load on the battery due to volume expansion at a time of charge and contraction at a time of discharge is still very large. Specifically, the copper foil used as collectors is greatly deformed, and internal short-circuiting is likely to occur at the time of the initial charge. Also, holes might be formed in the foil due to repetitive use, and seriously lower the level of security.